Type IV collagen has not yet been fully elucidated. Many molecules of type IV collagen are bound to each other and, in association with various components, form tissue basement membranes. Furthermore, type IV collagen is a major component of tissue basement membranes, and thus is also referred to as “basement membrane collagen”. Type IV collagen has six isomers: alpha 1 to alpha 6 chains, and one molecule of type IV collagen is composed of three alpha chains. The combination of three alpha chains varies depending on the tissue. Alpha 1 and alpha 2 chains occur in abundance in basement membranes derived from placenta, which are commonly found in the tissue basement membranes throughout the body. Basement membranes derived from kidneys have specificity and alpha 3 to alpha 6 chains are said to occur in abundance therein.
One molecule of type IV collagen is divided into three domains: the 7S domain located at the N-terminus and the central helical domain (TH), each having a triple-helical structure, and the NC1 domain having a non-helical structure located in the C-terminus region. When type IV collagen is extracted from the tissue of a living body, the domains obtained differ depending on the treatment method. In general techniques, triple-helical domains are obtained by pepsin treatment and NC1 is obtained by bacteria-derived collagenase treatment (Non-Patent Document 1).
Meanwhile, nephritis is broadly classified into “acute nephritis” and “chronic nephritis”, on the basis of symptoms, which are subclassified and variously named by specialists. Since nephritis usually refers to “chronic nephritis”, in the present invention, “nephritis” and “chronic nephritis” are used synonymously unless otherwise stated. Examples of “chronic nephritis” include, but are not limited to, so-called chronic nephritis, IgA nephropathy, minimal-change nephrosis, membranous nephropathy, and secondary nephropathy, such as diabetic nephropathy and hypertensive nephropathy. With respect to nephritis, 30,000 patients are newly introduced to dialysis every year, which causes a tremendous burden on the quality of life (QOL) of patients as well as to the national medical expenses (6,000,000 yen/year for each dialysis case). Although dialysis techniques are advancing, there are currently no pharmaceutical agents for treating nephritis directly by identifying nephropathy in the very early stage before dialysis is required. The major reason for this is that the mechanism of onset of nephritis has not yet been elucidated.
Furthermore, under the assumption that there exist antigens specific to nephritis, various antigen substances are currently under study around the world (Non-Patent Document 2). If antigens are identified, development of diagnostic agents or therapeutic agents for nephritis is expected, using the antigens as target substances. Goodpasture syndrome (anti-glomerular basement membrane (GBM) antibody nephritis in a narrow sense) is only one example in which antigens have been determined. It has been reported that the antigens are localized to 36 amino acid residues at the C-terminus of NC1 of the alpha 3 chain, and the amino acid sequences, from the N-terminus, 17 to 31 and 127 to 141 of the alpha 3 chain (Non-Patent Document 3). Each of the antigens is located on the alpha 3 chain. Furthermore, locations of 17 to 31 (15 amino acid residues) and 127 to 141 (15 amino acid residues) on the alpha 3 chain are being searched for from the chimera alpha 1/alpha 3 NC1.
Goodpasture syndrome, which is rare nephritis, belongs to “acute nephritis”, and typically, a serious condition occurs in about two weeks. Thus, a definite diagnosis must be immediately established. In order to establish the definite diagnosis, immunostaining of kidney biopsies is performed, and determination is made on the basis of the presence or absence of deposition of immunoglobulin IgG, which is an autoantibody to GBM.    Non-Patent Document 1: Extracellular Matrix IRL PRESS/OXFORD UNIVERSITY PRESS Oxford New York    Non-Patent Document 2: Jin to Toseki (Kidney and Dialysis) 2005 July Vol. 59 No. 1, Tokyo Igakusha    Non-Patent Document 3: J. Biol Chem (1993) 268, 26033-26036